Overview
Windows Messenger allows real-time communication with other contacts who are using instant messaging (IM) services including those provided by the Session Initiation Protocol (SIP) as offered by Microsoft Office Live Communications Server. Windows Messenger also provides connectivity to the Microsoft .NET Messenger service and is the IM client of choice for businesses in managed environments.

Windows Messenger 5.1 has been updated with changes in the following areas:
Improvements in file transfer functionality for users with multiple network connections (such as configurations involving both wired and wireless network connections or users of VPN connections).
Improvements in Tablet PC and laptop support. Specifically, this includes improved support for ink as well as improved performance when using power saving features in portable computers such as standby or hibernation.
Improvements to presence integration with applications, including better presence integration when applications are in full-screen mode (to avoid unwelcome IM interruptions) and also quicker refresh of presence information in applications such as Microsoft Outlook or Windows SharePoint services.
Improvements to the sign-in process, enabling a quicker sign in.
For customers using Microsoft Office Live Communications Server 2005, Windows Messenger 5.1 is the required client to take advantage of the following new features in Live Communications Server 2005:
Federation (sharing IM and presence between two or more organizations)
Advanced architecture (higher availability and failover support)
For more information about these features and Microsoft Office Live Communications Server 2005, click the Microsoft Live Communications Server home page link in the ‘related resources’ section.